AMD results when sulphide minerals are exposed to oxygen, water and aerobic bacteria. The sulphide minerals with which the invention is concerned include pyrite or marcasite FeS2, pyrrhotite FeS, chalcopyrite FeCuS2, bornite Cu5FeS4, sphalerite ZnS, arsenopyrite FeAsS, millerite NiS, pentlandite (FeNi)9S8, galena PbS, molybdenite MoS2, and other metal sulphide minerals which oxidize when exposed to oxygen, water, and aerobic bacteria.
At many mines, oxygenated precipitation water and oxygenated ground water pass through mine tailings, and mining waste, containing one or more of these sulphide minerals. At other locations, a mass or seam of acidity-causing sulphide minerals may have become exposed (as a result of workings, or even naturally) to the passage of oxygenated water therethrough. The exposed sulphide minerals can become oxidised, whereby water draining from these exposed bodies can comprise AMD or ARD, or an acidic metal-rich leachate, which is inimical to receiving aquatic environments.
Pyrite FeS2, for example, readily oxidizes in the presence of oxygen (air), water and bacteria in a complex manner according to a series of reactions which, in total, may be summarized in terms of the following simplified stoichiometric relationship: 
The resulting leachate is acidic, as indicated by the presence of sulphuric acid in the equation.
The iron in the pyrite changes from a reduced state (e.g. Fe++) to an oxidized state (e.g. Fe+++) in the hydroxide, and this change is in itself acid-producing; and by a feedback mechanism this change causes further oxidation and therefore further acidity.
These types of reactions are not confined only to iron. Where the mineral includes a metal other than iron, similar reactions which change the metal to a more oxidized state similarly produce further acidity.
The pH in the leachate waters which can be attributed to these mechanisms can be 3 or even lower in a typical real case. The acidity level that results from the oxidation of sulphide minerals is in most situations hazardous to local aquatic ecosystems.
Such acidity can develop wherever oxygenated moist conditions occur within materials composed of pyrite and other sulphide minerals. The acidity does tend to disperse eventually, as natural bases dissolve in the acid and by dilution as metals and other substances dissolve in the acid. However, this could take centuries, and relying on natural dispersion is unacceptable in most mine situations.
One approach to the problem of AMD and ARD has been to treat acid pond water (i.e water that has leached through an exposed body of pyrite or other mineral, and has collected in a pond) with continual additions of bases such as lime, Ca(OH)2. However, such treatment systems in general require a high degree of maintenance, which sometimes must be continued for many decades. And, no matter how effective that treatment system might be, in treating the acidic pond water, the water entering the pond will still be acidic, and the treatment will have to be continued. The cause of the acidity in the pond is the fact that the water entering the pond has passed through exposed pyrite, or other acid-producing mineral.
It is of course well-known that sulphide minerals lose their ability to cause acidity if the minerals are kept from becoming exposed to oxygen. Thus, sulphide minerals lying at the bottom of a lake, for example, can be harmless. Water at the bottom of a deep lake is normally devoid of oxygen, so that no acid producing reaction takes place. Sealing the acid-generating minerals by placing them at the bottom of a deep lake has been one approach to the problem of preventing the acidity-producing reactions from taking place.
As a way of preventing the acidity-producing reactions from taking place, one approach has been to seal the acid-generating minerals (pyrite or other) from exposure to atmospheric oxygen. This can be done, for example, though at great expense, by building a dam and flooding the exposed seam of pyrite. At even greater expense, the exposed portions of the acid-producing mineral can be excavated from the ground, and dumped in a deep lake (with unknown environmental consequences). Another approach has been to construct engineered covers that inhibit the diffusion of oxygen and/or the infiltration of oxygenated water. These techniques are extremely expensive and not always effective.
An aim of the invention is to shield a body of sulphide mineral from oxidation, or at least from oxidation at such a level as might lead to dangerous levels of acidity in leachate water. The invention basically involves providing an electrolytic cell, which de-oxygenates water passing therethrough, and involves positioning the cell in such a manner that the water entering the mass of sulphide mineral is the de-oxygenated water from the cell.
The principle of the electrolytic cell is known, in which two conductors are placed in an acidic electrolyte, the two conductors having different standings in the electro-chemical series; if a circuit is established between the two, under the right conditions a current will start to flow.
Free electrons become available at the electrodes, and reduce elemental oxygen to hydroxyl ions resulting in the production of alkalinity; in the absence of oxygen, hydrogen ions in the electrolyte reduce to hydrogen gas, which bubbles away. As a result, the pH at the cathode is raised.
When this system is constructed as a cover, placed over acidity-generating sulphide materials, much of the oxygen is removed from infiltrating oxygenated rainwater, and much of the oxygen diffusing through the cover is removed. The system can be operated as a galvanic system, using a sacrificial anode. Or the system can operate as an impressed-current system, which utilizes an external power source such as municipal AC power, or a DC solar cell, or a wind-powered generator.
It is an aim of the invention to utilize the principles of both the galvanic cell and the impressed current cell to form an electrochemical cover over acid-generating sulphide materials for the purpose of preventing the natural acidity-producing reactions from taking place.
It is an aim of the invention that the production of AMD/ARD be prevented, or at least be reduced to insignificance. It is an aim of the invention that the seepage water emerging through the underlying sulphide rich material should contain only an acceptably low concentration of acidity.
It is also an aim of the invention to provide a treatment system which can be left to operate by itself, with little or no maintenance. It happens sometimes that mine workings are not just closed, but are totally abandoned, with no resources (including no external power source) left for such things as cleaning up acidity. It is an aim of the invention to provide a treatment system which can be put in place economically in a mine in which such total abandonment is contemplated, or has occurred, being a treatment system which can be expected to keep acidity effectively under control indefinitely, with minimum or zero maintenance.